1. Technical Field
The present disclosure relates to a charged particle beam device, a control method for the charged particle beam device, and a cross-section processing observation apparatus.
2. Description of the Related Art
A charged particle beam device (also referred to as a focused ion beam device or the like) is a device which detects generated secondary electrons or the like in order to observe a microscopic image, or performs etching processing on a sample surface, by scanning the sample surface with a thinly focused ion beam. In addition, the charged particle beam device can build a system which obtains three-dimensional reconstructed image data by repeatedly performing processing of a very thin cross-section and a microscopic observation in combination with a scanning type electron microscope or the like (for example, JP-A-2009-204480).
In the charged particle beam device, the sample surface is scanned with a focused ion beam which is focused on the sample surface, by a deflector in a two-dimensional direction. A scanning range of the focused ion beam is indicated by a size of a scanning range on the sample surface, called a field of view (FOV), in cross-section processing or a microscopic observation. In addition, the FOV forms a two-dimensional plane including a plurality of pixels. A size of a single pixel in this case is a value corresponding to a unit scanning amount of the focused ion beam by the deflector (for example, JP-A-H06-295694 and JP-A-H07-201300).
The deflector which performs scanning with a focused ion beam is generally controlled by a computer. In this case, the computer generates a predetermined digital signal so as to control the deflector. In addition, the deflector is controlled by using a predetermined analog signal into which the digital signal is converted. In this case, a resolution of scanning or a scanning range of a focused ion beam by the deflector depends on the number of conversion bits of a digital/analog converter (DAC) which converts a digital signal output from the computer into an analog signal for controlling the deflector. For example, in a case of a 16-bit DAC, a range of values which can be converted is 0 to 65535 (or −32798 to 32767). In a case where, in the conversion range of 0 to 65535 of the 16-bit DAC, the range of 0 to 51200 is made to correspond to an FOV of which one side is 120 μm, the minimum analog output value (that is, a resolution) corresponding to an input value “1” of the DAC is calculated to about 2.34 nm (=120 μm/51200) as a scan length.
On the other hand, the above-described method of obtaining a three-dimensional reconstructed image data by repeatedly performing the thin cross-section processing and the microscopic observation is called a cut-and-see method, and a target object thereof is mainly a metal material or a semiconductor. In a case where a metal material or a semiconductor is used as an observation target object (that is, a sample), a thickness (that is, a slice amount) in cross-section processing per observation is typically of an order of about an nm order, and a length of one side of an FOV falls within about 10 μm in most cases. In contrast, a three-dimensional (3D) observation of a biological sample has recently attracted attention. Also in a case of the biological sample, a slice amount at one observation is required to have the fineness of an order of about nm. However, a size of a cell is large unlike metals, and thus an FOV with a length of about 100 μm to 150 μm is required to be used. For this reason, in relation to a size of a processing range (that is, a size of an FOV), a processing size having a large scaling ratio which is not considered in the related art is necessary, such as one side of 100 μm to 150 μm and a processing pitch (that is, a slice amount) of, for example, 2 nm. In the cut-and-see method, there is a case where one observation is performed for each slice, and also a case where an observation is not performed although slicing is performed, such as one observation being performed for every five slices.
As in the above-described example of a biological sample, in a case where an FOV has a relatively large size, and a slice amount has a very small value, the following problem may occur. In other words, for example, in a case of using the above-described 16-bit DAC, a resolution is about 2.34 nm when one side of the FOV is set to 120 μm. In this case, it is not possible to set a slice amount to, for example, 2 nm. In consideration of necessary quantization accuracy, the minimum value of an actual slice amount is limited to about fifty times the resolution, that is, 117 nm (=2.34 nm×50). In other words, if a slice amount is to be set to 2 nm by using the FOV of 120 μm, it is necessary to use a DAC having a resolution of about 1/50 or about 1/100 of the resolution of the 16-bit DAC. In this example, it is necessary to use a DAC having the number of bits of at least about 22 to 23 bits. However, this change of a DAC leads to a change of hardware and is thus problematic in that development cost increases.